Recent studies have emphasized the importance of innate immunity in Crohn's disease (CD) pathogenesis. Paneth cells are a key cellular arm of the innate immune system, secreting a variety of proteins involved in the host response to microbiota. Paneth cells also play an active role in regulating the adaptive immune response via their production of TNF, GM-CSF and other immunomodulatory peptides. The potential importance of Paneth cells to CD pathogenesis is also undrelined by the recent finding that NOD2, a gene that is strongly linked to CD susceptibility, is highly expressed in ileal Paneth cells. Our initial studies in SAMP1/Fc mice, a spontaneous model of ilietis resembling human CD, demonstrate marked expansion of Paneth and Intermedate cells in areas of disease activity. The central hypothesis of this proposal is that Paneth cells play a role in the pathogenesis of intestinal inflammation in CD either through the inappropriate or continued elaboration of pro-inflammatory mediators or by a unique molding of the intestinal microflora composition that is different from the unaffected population or both. The specific aims of the proposal are: Aim 1) Determine the mechanism by which the number of Paneth/intermediate cells is increased in the ileum of SAMP1/Fc mice. The contributions of luminal flora and inflammatory cytokines to the genesis of secretory cell hyperplasia and the association of the hyperplasia with genetic loci that determine disease susceptibility will be investigated; Aim 2) Determine whether expression of defensins and related peptides elaborated by Paneth/intermediate cells is altered in SAMP1/Fc mice and their role in the pathogenesis of ileitis. Expression of ileal Paneth cell defensins and related genes will be characterized at both the RNA and protein level. A targeted gene disruption strategy will be used to examine their role in disease pathogenesis; and Aim 3) Determine the functional role of Paneth and intermediate cells in the pathogenesis of ilietis. An epithelial cell lineage ablation approach will be used for these functional studies.